How can I force a read to hit the disk?

[Alan Stern]

My kernel module for Linux-2.6 needs to be able to verify that the media 
on which a file resides actually is readable.  How can I do that?

It would certainly suffice to use the normal VFS read routines, if there
was some way to force the system to actually read from the device rather
than just returning data already in the cache.  So I guess it would be 
enough to perform an fdatasync for the file and then invalidate the file's 
cache entries.  How does one invalidate a file's cache entries?  Does 
filemap_flush() perform both these operations for you?

TIA,

Alan Stern

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